Implanted pacing systems may be used to deliver cardiac resynchronization therapy (CRT) or to otherwise pace the heart. When the heart is paced in the left ventricle (LV), for example, there may be unwanted stimulation of the phrenic nerve that causes contraction of the diaphragm. Unintended phrenic nerve activation (an unintended action potential propagated in the phrenic nerve) is a well-known consequence of left ventricular pacing. The left phrenic nerve, for example, descends on the pericardium to penetrate the left part of the diaphragm. In most people, the left phrenic nerve runs close to the lateral vein. The unintended phrenic nerve activation may cause the diaphragm to undesirably contract. Unintended phrenic nerve activation may feel like hiccups to the patient. Such unintended phrenic nerve activation can occur when the electric field of the LV pacing lead is proximate to the left phrenic nerve and is at a stimulation output that is strong enough to capture the nerve.
Unintended phrenic nerve activation may vary from patient to patient. One reason for this variance is that the anatomic location of the phrenic nerve can vary within patients. Additionally, the veins are not always in the same location with respect to the ventricle and the nearby passing nerve. Also, the selected vein in which to place a cardiac lead for a prescribed cardiac therapy may vary. Furthermore, the location of the lead (e.g. LV lead) within the vein may vary.
Cardiac therapies may be delivered using different pacing configurations and different stimulation parameters. Examples of pacing configurations include LV bipolar, LV to can, and LV to RV (right ventricle) also referred to as “extended bipolar.” Examples of stimulation parameters include the amplitude (e.g. voltage) and pulse width. The pacing configuration or the stimulation parameters of a therapy may be modified in an effort to avoid phrenic nerve stimulation.
For example, an implantation procedure may be modified to avoid phrenic nerve capture. For example, the LV pacing electrodes may be repositioned to capture the LV for a pacing therapy such as CRT while avoiding phrenic nerve capture, or the clinician may decide not to implant an LV pacing electrode but rather rely on other pacing algorithms that do not pace the LV.
Although phrenic nerve stimulation is commonly assessed at implant, unintended phrenic nerve activation caused by phrenic nerve capture during pacing may first appear or worsen post-implant for a variety of reasons such as reverse remodeling of the heart, lead micro-dislodgement, changes in posture, and the like. Therefore, special office visits after implant may be necessary or desirable to reprogram the device to avoid phrenic nerve stimulation.